This invention relates generally to packaging arrangements for energy dissipating devices and, more specifically, to a packaging arrangement having improved thermal characteristics, heightened resistance to the penetration of moisture, and reduced thermo-mechanical stressing. The arrangement of the present invention is especially well-suited for packaging electrical, electronic and microelectronic components, and can be used with single components, multiple components, or complete circuits (e.g., printed circuit boards).
The use of plastic encapsulation as a packaging technique for electronic devices is well known. Problems of corrosion and inadequate thermal performance have been noted in plastic encapsulated devices. More recently, with the advent of Very Large Scale Integrated (VLSI) Technology, the additional problem of thermo-mechanical stressing has arisen as a major concern to packaging designers.
Consideration of plastic encapsulation techniques currently in use raises questions as to the basic soundness of this packaging method for all applications. The Transfer Molding Process commonly used allows molten plastic to flow around and past the device and the delicate bond wires, before eventual cross-linking and cure. The close contact between the somewhat delicate circuitry on the device and the encapsulant does not appear to be as benign as was once presumed. Furthermore, the adhesion at the plastic/lead frame interface has always been a source of concern, as has the relatively poor thermal performance of the device which results from the forced reliance on the low thermal conductivity plastic for transporting the major portion of energy dissipated at the device. Consideration of these problems reveals the need to identify more effective techniques for protecting the subject device both during and after completion of the packaging process, and for transferring heat from the packaged device to its environment.
An object of the present invention is to provide a packaging arrangement for energy dissipating devices which has improved thermal characteristics.
Another object of the present invention is to provide a packaging arrangement for energy dissipating devices which offers a greater degree of mechanical protection for the packaged device and associated connections.
Yet another object of the present invention is to provide a packaging arrangement for energy dissipating devices which is especially well-suited for use with preformed thermoplastic packaging elements which are used to form an outer shell or casing around the packaged device.
Still another object of the present invention is to provide a packaging arrangement for energy dissipating devices which incorporates thermal elements which are specially designed, or which can be easily modified or adjusted, to regulate the transfer or flow of heat from the packaged device.
Still another object of the present invention is to provide a packaging arrangement for energy dissipating devices in which the thermal elements used to regulate heat transfer from the device form a sealed inner enclosure around the device to provide increased mechanical protection of the device and to reduce the migration of moisture from the environment to the area immediately surrounding the device.
Still another object of the present invention is to provide a packaging arrangement for energy dissipating devices which includes one or more heat-flow modifiers placed within the package to control the heat flow rates in major flow paths so as to optimize the thermal performance of the package for given sets of operating conditions.
Still another object of the present invention is to provide a packaging arrangement for electronic devices which is especially well-suited for use with high pincount devices.
These and other objects of the invention are attained in a packaging arrangement which comprises first and second thermal elements extending in generally planar directions adjacent respective first and second sides of the energy dissipating device, a plurality of electrically conductive leads extending from the device to the exterior of the packaging arrangement, means for electrically isolating the leads from the thermal elements and for effecting a seal between the leads and thermal elements, an outer casing surrounding the first and second thermal elements, and means for effecting a seal between the outer casing and the leads. At least one of the first and second thermal elements is in thermal communication with the packaged device, and the other of the elements is in thermal communication with either the device or the other thermal element. The plurality of leads extend between the first and second thermal elements, and at least one of the elements extends adjacent a portion of the leads and is in thermal communication with these lead portions. The leads extend through the outer casing to the device environment.
A preferred embodiment of the present invention further comprises a heat-flow modifying element adjacent at least one of the first and second thermal elements for regulating the flow of heat therefrom. In an especially preferred embodiment of the invention, the thermal conductivity of the heat-flow modifier may be adjusted or varied, depending upon internal and external operating conditions. The modifier may comprise a metallic element. One or more openings or channels may be provided in the element to accommodate a heat flow regulating medium. Alternatively, the heat-flow modifier may comprise a cavity of specified dimensions. The cavity may be left empty so as to form an air gap, or may be filled with a heat flow regulating substance (e.g., an expanded foam) to provide the required thermal resistance necessary for optimal energy distribution in the packaging arrangement. At least a portion of an interior surface of the cavity may be lined with a thin metal foil to provide resistance to moisture penetration of the cavity.
In a preferred embodiment of the invention, a portion of the second thermal element contains a material having a high energy of transformation from solid to liquid, which portion is situated in close proximity to a surface of the packaged device. The portion of the second thermal element close to electrically active regions of the device will be coated with a thin semiconductor grade dielectric material for assured electrical isolation. Alternatively, the second thermal element may comprise a metallic sheet and a thermally conductive, electrically insulated elastomeric material disposed between the sheet and the device. The elastomeric material preferably includes an outer, relatively rigid elastomeric crust and an inner, relatively soft elastomeric core.
In an especially preferred embodiment, the thermal elements substantially surround the device to form an inner casing or enclosure around the device to provide a degree of mechanical protection and to inhibit the transport of moisture from the package environment to the area immediately surrounding the device. The outer casing may be formed by an encapsulating technique, but is preferably preformed of a thermoplastic material in at least two sections, including a base and a cap. In this embodiment, one of the thermal elements is mounted in, or formed integrally with, the base section of the outer casing, while the other thermal element is similarly mounted in or formed with the cap section of the casing. After the device is mounted in the base section and appropriate connections are made, the base and cap sections are sealed together to form the completed package. The base and cap may be sealed by a fusion bonding technique, or by injecting a molten thermoplastic material into a joint formed by adjacent surfaces of the base and cap.
In yet another especially preferred embodiment of the invention, a seal between the outer casing and the leads is formed by a seal assembly which comprises a relatively soft, inner elastomeric core surrounding each respective lead, and a relatively rigid, outer elastomeric crust substantially surrounding the relatively soft inner core. The seal assembly is compressed and cured in situ, after which the base and cap are joined so as to establish a tight seal around each lead.
The means for electrically insulating the leads from the thermal elements preferably includes a thermoplastic material or preformed film disposed between the leads and the thermal elements. When the preformed encasement technique is used, the thermoplastic film or material is preferably formed from a thermoplastic which is the same as or compatible with the thermoplastic used to form the base and cap sections of the outer casing so that at least some bonding will occur between the film or material and one or both of these sections when the package casing is formed and the package is sealed.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.